One of the major environmental problems confronting certain areas including major cities, in the United States and other countries is of atmospheric pollution associated with the emission of gaseous pollutants from automobiles, including both evaporative emissions and exhaust gas pollutants. This problem may be acute in major metropolitan areas such as Los Angeles, Calif. where atmospheric conditions in combination with large numbers of automobiles create appropriate conditions for aggravated air pollution.
In addition to evaporative emissions from the gasoline tanks of the vehicles and emissions from product terminals and tankers, hydrocarbons are also found as unburned or incompletely burned hydrocarbons in the exhaust emissions together with nitrogen oxides (NOx) and carbon monoxide (CO), all of which contribute to air pollution.
The composition of motor gasolines commercially sold for normal road vehicle use in certain areas of the United States is now restricted by Federal and, in some cases, by State regulations. The California Air Resources Board (CARB) has established a legal reference framework for the sale of motor gasolines in California which is intended to reduce the severity and extent of air pollution in that State from gasoline powered road vehicles and other mobile sources fueled with motor gasoline. The CARB regulations for Clean Burning Gasolines (CBG) are found in Title 13 of the California Code of Regulations, principally in Sections 2260 et seq., with Sections 2260 to 2270 dealing with the predictive model (PM) established under the regulations. Reference is made to these regulations as well as to the document “California Procedures for Evaluating Alternative Specifications for Phase II Reference Gasolines Using the California Predictive Model”, for details of the model and the test procedures to be used in conjunction with it. The present invention deals mainly with gasolines which either conform to the California regulations or which provide emissions no higher than those permitted under the current regulations. The US federal regulations are set by the Environmental Protection Administration (EPA) which has established initially a simple predictive model (the Simple Model) and, subsequently, a Complex Predictive Model (the Complex Model) for predicting vehicle evaporative and exhaust emissions.
The CARB Regulations regulate the composition of road vehicle motor gasolines in two ways. A simple prescriptive compositional standard for CBG may be followed but as an alternative, a fuel may be evaluated by the predictive model with the requirement that exhaust emissions should be no higher than those resulting from a fuel which conforms to the compositional specifications. The predictive model ultimately sets limits on vehicle emissions according to various compositional parameters, for example, sulfur, olefins and aromatics contents as well as by reference to distillation characteristics including the distillation points including the 10%, 50% and 90% distillation points (T10, T50, T90) of the gasoline. The “D-86 Distillation Point” refers to the distillation point obtained by the procedure identified as ASTM D 86-82, which can be found in the 1990 Annual Book of ASTM Standards, Section 5, Petroleum Products, Lubricants, and Fossil Fuels. Unlike the EPA model, the CARB predictive model has no specification for evaporative emissions, as commonly measured by the Reid Vapor Pressure (RVP) method, confining itself to exhaust emissions produced on the combustion of the gasoline fuels. “Reid Vapor Pressure” (RVP) is a pressure determined by a conventional analytical method for determining the vapor pressure of petroleum products. In essence, a liquid petroleum sample is introduced into a chamber, then immersed in a bath at 100° F. (37.8° C.) until a contstant pressure is observed. Thus, the RVP is the difference, or the partial pressure, produced by the sample at 100° F. (37.8° C.). The complete test procedure is reported as ASTM test method D-323-89 in the 1990 Annual Book of ASTM Standards, Section 5, Petroleum Products, Lubricants, and Fossil Fuels. The EPA Complex Model provides a predictive model for evaporative effects of various compositions and it appears that consideration of evaporative emissions is a relevant factor since a review of recent CARB emission inventories indicates that evaporative emissions contribute about 30% to total hydrocarbon emissions.
CBG specifications set by CARB set absolute limits on certain gasoline parameters such as sulfur content and, in addition, permit the compositions of pump gasolines to be varied within these absolute limits either by composition on a per gallon or an averaged basis or by reference to the Predictive Model. The compositional specifications are as shown in Table 1 which follows:
TABLE 1CBG Gasoline SpecificationsPropertyCapsPer GallonAverageSulfur, ppm804030Benzene, wt %1.21.00.8Aromatics, vol %302522Olefins, vol %1064Oxygen, wt %2.71.8/2.2RVP, psi7.07.0T50, ° F.220210200T90, ° F.330300290
The oxygenate content, set at a maximum of 2.7 wt % in Table 1 above (as oxygen, corresponding to about 10 wt % or more, e.g., 12 wt % as actual oxygenate), may be increased to 3.5 percent under a proposal being considered by CARB. See Notice of Continuation of Public Hearing to Consider an Amendment to the California cleaner Burning Gasoline Regulations by Increasing the Cap Limit for Oxygen from 2.7 to 3.5 Percent by Weight, Hearing set for 10 Dec. 1998, Sacramento, Calif. The oxygenate content may be varied under the predictive model as long as the fuel results in emmissions no worse than those resulting from the average/per gallon fuel selected as the basis for comparison. The federal RFG oxygenate requirement has to be observed year round in the California areas covered by federal RFG (Los Angeles, Sacramento and San Diego) and in addition, a minimum 1.8 wt. pct. oxygen is required in certain areas in California during the winter for CO control (Los Angeles Metro area, Imperial County and for the next two years only Fresno and Lake Tahoe).
Proposals have been made in the past for the development of motor gasolines which produce lower amounts of gaseous pollutants on combustion, notably U.S. Pat. Nos. 5,288,393; 5,593,567; 5,653,866 and 5,837,126, Jessup et al., assigned to Union Oil Company of California. According to the Jessup patents, the principal factor influencing the hydrocarbon and/or CO exhaust emissions is the 50% distillation point (T50) which is held at a maximum value of 215° F. (102° C.) with the hydrocarbon and CO emissions progressively decreasing as T50 is reduced below this value. It is stated that preferred fuels have T50 of 205° F. (96° C. or less) with best results being attained with T═being below 195° F. (91° C.) NOx emissions are stated to be minimized or reduced in dependence upon RVP as the principal factor with T10 as a secondary factor. NOx emissions are stated to decrease as RVP is decreased to 8.0 psi (0.54 atm) or less, preferably to 7.5 psi (0.51 atm) or less with an expressed preference for values below 7.0 psi (0.48 atm). The 10% distillation point and the olefin content are stated to be of secondary importance with respect to NOx emissions with olefin contents below 15 vol % providing some reduction in NOx emissions, preferably with zero content of olefins. The 10% point (T10) is stated to provide some reduction in NOx emissions at values below 140° F. (60° C.). Although decreases in olefin content are likely to be more acceptable to the refiner than decreasing T10, it is stated that the olefin content will be the secondary variable providing the most flexibility to the refiner in altering gasoline composition to reduce NOx emissions. The conclusion is expressed that best results are attained when both the olefin content is below 15 vol %, preferably 0, and the RVP is no greater than 7.5 psi (0.51 atm) with the T10 preferably being below 140° F. (60° C.). A number of gasoline compositions are set out in the Jessup patents together with calculated and experimental emission data for such fuels.
While the predictive models utilized by the EPA and CARB provide a comprehensive framework for evaluating the potential effects of variations in motor gasoline composition, further development work has shown that it is possible to control emissions effectively—and even to reduce emissions—below current levels while giving the refiner additional flexibility in the compositions of the gasoline's. This is based on a number of considerations including the sensitivity of emission parameters (toxics, hydrocarbons, CO and NOx) as related to the variables in the CARB predictive model (oxygenate content, sulfur, T90, T50, aromatics, olefins, benzene and RVP). Toxics and total hydrocarbone (THC's) in the CARB predictive model are very sensitive to T50 values above 210° F. but these increases can be offset by adjusting other variables including an evaporative factor such as RVP, as well as decreased sulfur. If appropriate adjustments in the compositinal parameters are made it may possibly permit increased olefins levels at the same time, which is a useful consideration for refiners which utilize a significant amount of FCC gasoline in the final blend. While certain compositional variations may fall within existing regulatory limits, certain others fall outside current limits but again, have the potential of providing lower total emissions than those resulting from compositions which are in accordance with current limits. The potential for providing lower total emissions (evaporative and exhaust) indicates that by including an evaporative parameter to the predictive model it may be possible when offsets from other properties are factored in, to provide reductions in hydrocarbon emissions sufficient to offset the increases resulting from an increase in T50. Further, the addition of an evaporative parameter to gasoline compositions may be prudent in view of the finding that evaporative emissions approximate to some 32% of total hydrocarbon emissions with projections showing an increase after the year 2000. Reductions in the RVP would provide improved flexibility in manufacturing and blending operations for pump gasoline without environmental harm. In fact, the benefits from including an evaporative parameter in the CARB model could be significant. Each 0.1 psi reduction in RVP provides hydrocarbon emission reductions equivalent to a reduction of 2° F. (1° C.) in T50.
The effects of including an evaporative parameter in the fuel certification model and reducing RVP from 7.0 to 6.6 psi in the predictive model were investigated with respect to the CARB predictive model by systematically varying fuel properties within the following ranges: T50 210-220° F., T90 295-330° F., sulfur 5-35 ppm, aromatics 12-28 vol %, and olefins 2-10 vol %. Oxygen was held at 2% and benzene at 0.7%. The percent of fuels tested which meet all CARB emission contraints are summarized in Table 2 below:
TABLE 2T50,BaseRVP AddedRVP Added° F.ModelExhaust and EvaporativeEvaporative Only21048%81%63%21240%77%61%21431%71%57%21621%64%52%21813%52%44%220 5%38%34%
The evaporative parameter here is based on the CARB revised draft “Outline of an Evaporative Modeling Proposal”, revised 6 May 1998, original distributed for public consultation 5 Feb. 1998.
From these results it is clear that the addition of an evaporative factor which can account for the beneficial effect of reducing RVP has the potential for improving actual emission levels. In the base model it is difficult to increase T50 beyond 214° F. unless sulfur and aromatics are very low. With the RVP factor added, however, there is a significant increase in flexibility and T50 levels of 220° F. and higher are possible. The use of T50 values above 215° F. therefore becomes possible, with values in the range of 215° F. to 220° F. representing an area in which there is significant potential for the formulation of gasolines with acceptable levels of total emissions according to the standards now prevailing in California. The greatest flexibility is provided when the exhaust RVP effects are also added due to the added NOx benefits.